(1) Field of the Invention
The present invention relates generally to acoustic sensors, and in particular to a laser vibrometer acoustic sensor for use in underwater applications.
(2) Description of the Prior Art
Sonar systems traditionally use an array of pressure-sensing hydrophones to detect underwater sound. The individual hydrophones (or array elements) are configured in a linear, planar, or conformal grid and then the output voltage from each hydrophone is summed. Fundamentally, the hydrophone converts the underwater acoustic pressure to a calibrated voltage. Array sampling theory requires that the separation between the array elements be no greater than one half the acoustic wavelength at the array's upper design frequency.
Pressure sensors, or hydrophones, are commonly used in these acoustic arrays. These sensors are made of piezoelectric ceramic materials which convert mechanical stresses in the ceramic due to an incident acoustic pressure into a calibrated output voltage. More recently, acoustic velocity sensors have been used in acoustic arrays. These velocity sensors also use ceramic's piezoelectric properties to convert acoustic particle velocity to an output voltage. Thus, the acoustic frequencies utilized by the array are limited by the sensor spacing
Scanning laser vibrometers are well known in the prior art. These include commercially available systems such as the Polytec® Model PSV-100 Scanning Laser Vibrometer System (SLVS). This system can sample a grid of 512 by 512 points, with each grid point having a spot size of 0.0004 inches. The output of the SLVS provides an indication of the velocities of an object at the grid points to indicate vibrations of the object.
It has previously been proposed to use this technology for underwater acoustic detection.
Walsh et al. in U.S. Pat. No. 6,188,644 teach a photon transducer system is provided for obtaining information on acoustic signals within a fluid environment. The photon transducer system uses a laser-based Doppler interferometer located within a pressure release surface. The pressure release surface is formed by generating a gas pocket in the fluid, creating a boundary layer between the laser light source and the surrounding fluid. Laser light is reflected from the boundary and is detected by the interferometer to obtain the Doppler velocity of the pressure release surface. The pressure incident on the boundary can be determined from the measured velocity, providing information on the incident acoustic pressure.
Glenning et al. in U.S. Pat. No. 6,349,791 teach a submarine bow dome acoustic sensor assembly that comprises an outer hull bow portion, an inner pressure hull wall extending athwartships and in conjunction with the outer hull bow portion defining a free-flood compartment, and an acoustic bow panel disposed in the compartment and connected to the pressure hull wall by acoustically isolating supports. A laser scanner is disposed in the compartment and is oriented so as to project a laser beam onto a surface of the acoustic bow panel, and a sensor is disposed in the compartment and oriented so as to receive reflections of the laser beam off the acoustic panel and to transmit data from which a position of a sound generating source can be determined.
Antonelli et al. in U.S. patent application Ser. No. 11/070,400 teach an acoustic sensor used in underwater applications. The sensor includes a reflective material adhered to one side of a structure, such as an outer submarine hull or any marine vessel hull. A laser interferometer is placed on the side of the structure with the reflective material. The laser interferometer sends a plurality of laser beams, in sequence or all at one time, to a plurality of points across the retro-reflective material. The laser beams reflect back to the interferometer, which captures the reflected beams using receiving optics. The phase modulation of the reflected laser beams is compared to a reference laser beam within the interferometer to obtain the vibration velocity characteristics of the hull surface structure. Since the reflective material is adhered to the structure, the structure vibration is the same as the vibration of the reflective material. From this vibration, the acoustic pressure associated with the structure may be calculated.
Walsh et al. provide for measurement of characteristics of a pressure release surface on the exterior of an underwater vehicle. It is suggested that various characteristics of the disclosed system make it unsuitable for use as an underwater acoustic sensor. The laser interferometer is not capable of scanning the boundary surface to conduct beamforming. The boundary surface is not sufficiently stable to clearly determine acoustic vibrations. Furthermore, the laser interferometer is not insulated from vehicle vibrations.
Glenning et al. use a scanning laser for measurement of vibrations impinging on a bow panel located in a free flood area of an underwater vehicle. During a test of a similar system, it was found that at oblique angles, the incident sound field insonified both the panel and the acoustic lens of the underwater housing containing the laser. This distorted the time series measurements made on the surface of the panel. It was also found that water particles in the path of the laser were insonified further distorting the acoustic waves.
Antonelli et al. disclose an air/water acoustic window, but they don't disclose the use of vibration isolation for the window and the laser vibrometer. As above, it has been found that this lack of isolation interferes with the reception of external acoustic signals.
In view of the prior art, there is a need for an acoustic sensor system that does not have the limitations imposed by prior art ceramic devices. There is a further need for a sensor that is isolated from structure borne noise and capable of receiving undistorted acoustic signals from normal and oblique acoustic sources.